Adding chlorine is not enough: if the pH is wrong, the disinfectant is not working

According to the World Health Organization’s swimming pool guidelines, when chlorine gas or hypochlorite is added to water, hypochlorous acid (HOCl) is formed. Hypochlorous acid dissociates in water into H⁺ and OCl⁻ (the hypochlorite ion).

The degree of dissociation depends on pH — and much less on temperature. Dissociation is minimal below pH 6; between pH 6.5 and 8.5 a change occurs from undissociated hypochlorous acid to nearly complete dissociation.

And the decisive sentence: hypochlorous acid is a much stronger disinfectant than the hypochlorite ion. The same reading of “free chlorine 1.5 mg/l” therefore does not give the same protection at pH 7.2 and at pH 8.2.

According to WHO, the pH value of swimming pool water must be controlled to ensure efficient disinfection and coagulation, to avoid damage to the pool fabric and to ensure user comfort. The recommended range: 7.2 to 7.8 for chlorine disinfectants, and 7.2 to 8.0 for bromine-based and other non-chlorine processes.

For free chlorine, WHO recommends that acceptable levels continue to be set locally; but in public and semi-public pools these should not exceed 3 mg/l, and in hot tubs 5 mg/l.

WHO also states that pool water should be monitored for microbial parameters: heterotrophic plate count, thermotolerant coliforms or E. coli, Pseudomonas aeruginosa and Legionella.

Alkalinity is not a health parameter in itself; it determines the stability of pH. In WHO’s definition, alkalinity is a measure of the alkaline salts dissolved in the water: “The higher the alkalinity, the more resistant the water is to large changes in pH in response to changes in the dosage of disinfectant and pH correction chemicals.”

But too much is also a problem: WHO notes that if the alkalinity is too high, it can make pH adjustment difficult. The aim is not “more is better” but staying within a range.

The M alkalinity, P alkalinity and total alkalinity in our catalogue are different end points of the same titration; together they describe the buffering capacity of the water.

Temperature affects three things at once: how the disinfectant behaves, how fast microorganisms grow, and user comfort. The Turkish Regulation on the Health Principles and Conditions Applicable to Swimming Pools sets it numerically in Annex 2: 26–28 °C for indoor pools and 26–38 °C for outdoor pools.

The same annex gives values for colour and turbidity as well: colour 10 units as Pt/Co, turbidity 5 units as SiO₂ or Jackson units.

The regulation also requires free chlorine to be measured at the poolside — because the chlorine value changes before the sample reaches the laboratory.

The regulation defines a pH range of 6.5 – 7.8 for pool water; compliance on the report is judged against that range. The 7.2 – 7.8 range described above comes not from the regulation but from the chemistry of chlorine: as pH rises the proportion of hypochlorous acid falls, and the same free chlorine figure gives less protection.

So a pool measured at pH 6.8 complies with the regulation while sitting outside the ideal window for chlorine efficacy. The two pieces of information do not replace each other: the report states compliance, the chemistry states how well protected the water actually was that day.

Alkalinity is the buffer behind all of this, and the regulation bounds it at 30 – 180 mg/L CaCO₃. Too low and pH swings through the day; too high and pH becomes hard to bring down. Combined chlorine is limited to 0.2 mg/L at most — that sharp “chlorine smell” at a pool is in fact the signature of combined chlorine, not free chlorine.